The Life of the Plant. 177 
gum-like substances, tannin, pigments, acids, alkaloids, wax, 
pectine, &c. Other substances, known as degradation-pro- 
ducts, are produced only by subsequent changes in the sub- 
stance of organised structures ; such are the mucilage of 
quince-seeds and linseed, and perhaps also lignine and cork 
(see p. 23). - | 
The object of the processes of assimilation and meta- 
stasis, the nutrition of cells already existing and the formation 
of new ones, is at present known only in its most general 
features. The little that is known consists of the laws of 
cell-formation already mentioned, and of the fact that the 
cell-wall grows by zztussusception, that is, by the formation 
of fresh molecules between those already in existence,! 
REPRODUCTION, 
The great office assigned to vegetation of covering the 
surface of the earth with forms of organic hfe, and trans- 
forming inorganic into organic substances in order to support 
the life of man and other animals, requires that plants be 
endowed in a remarkable manner with the power of repro- 
_ duction, that is, of producing individuals of the same species, 
in order that the short duration of all earthly life may not 
result in the world being soon deprived of its covering of 
vegetation, and all life perishing in consequence. For this 
purpose, particular cells of the plant begin, at definite times, 
an independent growth, and thus carry on a life of their 
own, often not in connection with the parent plant. Such 
cells are called reproductive cells, and the plant which re- 
sults from one of them constitutes a distinct generation. ‘The 
successive generations are only rarely, as in some Algee and 
Fungi, alike ; as a rule, generations of a different kind follow 
1 [This sentence is somewhat altered from the original, where a dif- 
ferent definition is given to the term ‘intussusception’ from that usually 
adopted, and previously employed in this work.—ED. ] 
N 
